Method for producing a component by means of powdery starting material and extractor suitable therefore

ABSTRACT

According to a known method for producing a component by means of a powdery starting material, powder is mixed with auxiliaries comprising binding agents, whereby a free-flowing mass is produced. A green body is produced from the mass by means of powder injection moulding. A portion of the auxiliaries is extracted by heating the green body in a container and exposing said body to a stream of solvents. The green body is sintered, whereby the component is produced. The aim of the invention is to enable fast and gentle extraction for releasing green bodies which are produced by means of powder injection moulding. According to the invention, the extraction comprises a heating-up phase during which the temperature of the green body is permanently or gradually increased and the green body is overflown by solvent in the form of a compressed, supercritical treatment gas. An extractor that is suitable for carrying out said method is characterised by a pressure sealed housing that is provided with at least one lateral limiting wall and encloses a treatment chamber for receiving a carrier element which is loaded with a plurality of green bodies. Said housing is provided with a lock-up load and removal opening for the green bodies. Said opening is configured in the lateral limiting wall as a passage for the carrier element.

[0001] The invention relates to a method for producing a component froma pulverulent starting material by mixing the starting powder togetherwith auxiliaries, comprising binders, resulting in the formation of afree-flowing mass, after which a preform is produced from the mass bymeans of powder injection molding, then some of the auxiliaries areextracted in that the preform is heated in a tank and exposed to asolvent stream, after which the preform is sintered, thus giving rise tothe component.

[0002] Furthermore, the invention relates to an extractor for carryingout the method.

[0003] Powder injection molding is preferably used for the production ofgeometrically complex components in large numbers. Especially materialsthat are mechanically difficult to work such as, for instance, hardmetals, ceramics and graphite, are easy to process with this method.

[0004] The process sequence for powder injection molding is described inthe product information publication titled “Powder MetallurgicalInjection Molding—an advanced manufacturing method” issued by SchunkSintermetalltechnik GmbH (imprint: 52.02/1996). Here it is explainedthat powder injection molding can be broken down into several steps.Pulverulent starting material—as a rule metallic or ceramic powder orelse graphite—is mixed with thermoplastic binders and plasticizers in afirst process step to form a free-flowing mass. This mass can beprocessed on injection molding machines into preforms whose contours areclose to the final shape. In order to remove the binders andplasticizers from the preform, it undergoes a so-called extraction orpyrolysis in a subsequent process step. During the extraction, theabove-mentioned auxiliaries are chemically dissolved in a solvent andremoved from the preform, whereas they are thermally broken down in thecase of pyrolysis. The resultant brown blank is sintered in the nextprocess step to form a metal, ceramic or graphite object.

[0005] The binders and plasticizers not only facilitate the processingof the mass but they also give the preform the necessary dimensionalstability for its further processing. In order for the preform to retainits shape, the removal of these auxiliaries must be carried outgradually and gently. This applies especially to the extraction of thebinders and plasticizers from preforms that have been made by means ofpowder injection molding. The reason is that, with this method, thepreforms are generally de-molded before the extraction step and areconsequently unsupported —free-standing or, for example, lying in a sandbed —when they are exposed to the solvent stream. This is why bindersystems are used that are solid at room temperature and that can givethe preforms the necessary dimensional stability at this temperature.The removal of the binders and plasticizers by means of pyrolysis—depending on the wall thickness of the preform —takes several days;extraction by means of solvents also calls for long treatment times.Moreover, solvents and some of the breakdown products of conventionalbinder systems have an adverse impact on the environment.

[0006] In order to conduct an extraction, a large number of the preformsfrom which the binder is to be removed are placed into an extractiontank as a bath and treated with the extraction agent.

[0007] Therefore, the invention is based on the objective of creating amethod for extracting binders that allows quick and, at the same time,gentle binder removal from preforms that have been produced by means ofpowder injection molding.

[0008] Moreover, the invention is based on the objective of providing anextractor that is suitable for carrying out this method.

[0009] Regarding the method, this objective is achieved on the basis ofthe method mentioned above in that the extraction comprises a heating-upphase during which the temperature of the preform is continuously orincrementally increased and a solvent in the form of a compressed,supercritical treatment gas flows around the preform.

[0010] A solvent in the form of a supercritical treatment gas is usedfor the extraction of the auxiliaries from the preform. The treatmentgas flows around the preform, dissolving auxiliaries —such as bindersand plasticizers —which are subsequently discharged from the tanktogether with the treatment gas. In this process, the preform becomesmore and more porous as a result of the removal of the auxiliaries, sothat the treatment gas can also flow through the porous regions of thepreform. The tank is a so-called extractor in which an excess pressurecan be built up.

[0011] The supercritical treatment gas that is used as the-solvent ischaracterized by a high density which, in turn, improves the solventaction vis-a-vis the auxiliaries that are to be dissolved from thepreform. The high density is achieved in that the supercriticaltreatment gas is kept in a temperature-pressure regimen above itsspecific critical temperature and pressure values.

[0012] The extraction comprises a heating-up phase during which thetemperature of the preform and consequently also the temperature of thetreatment gas flowing around it is raised. The temperature is raisedcontinuously or incrementally. The special action of this method isexplained in greater detail below:

[0013] The auxiliaries contained in the preform are generally present indiffering fractions that can vary, for example, in terms of theirthermal properties —such as their melting temperature —or in theirchemical and physical properties —such as their solubility in thetreatment gas. An auxiliary fraction that has been removed from thepreform by dissolution in a treatment gas or that has been liquefiedthrough heat no longer contributes to the dimensional stability of thepreform. By slowly raising the temperature of the preform during theextraction, it is achieved that the auxiliary fractions, as a functionof their solubility in the treatment gas and of their meltingtemperature, are consecutively removed from the preform. First of all,the auxiliary fractions that are soluble at a low temperature areextracted, and those that do not dissolve as readily are extracted asthe temperature rises. Here, in the ideal case, the auxiliary fractionsare removed by dissolution in the treatment gas before their individualmelting temperatures are reached.

[0014] The solubility of the auxiliaries in the supercritical treatmentgas increases as a function of the temperature. If extraction werecarried out at a low temperature, the removal of the auxiliaries wouldtake a long time, but on the other hand, if extraction were conducted ata high temperature, the dimensional stability of the preform would belost due to rapid removal or softening of the auxiliaries. Only raisingthe temperature gradually (continuously or incrementally) allows agentle and, at the same time, rapid removal of the auxiliaries from thepreform.

[0015] The essential precondition for this is the use of binder systemsthat are solid at room temperature, which allows such a gradual raisingof the temperature without the preform losing its dimensional stability.Therefore, the method according to the invention can be used especiallyadvantageously for the extraction of binders and plasticizers from thosepreforms that were made by means of powder injection molding and that,as a rule, had already been de-molded and were unsupported during theextraction, as a result of which they have to have sufficientdimensional stability on their own.

[0016] The temperature course during the extraction depends essentiallyon the thermal and chemical properties of the auxiliaries employed aswell as on the geometrical dimensions of the preform. It has provenadvantageous to establish a heating rate during the heating-up phase inthe range from 0.1° C./min to 5° C./min [0.2° F./min to 9° F./min],preferably in the range from 0.5° C./min to 2° C./min [0.9° F./min to3.6° F./min]. A heating rate in this range generally brings about arapid and gentle extraction of the auxiliaries.

[0017] This applies equally to a mode of operation in which, during theheating-up phase, the temperature of the preform is raised to the rangefrom 20° C. to 150° C. [68° F. to 302° F.], preferably from 30° C. to130° C. [86° F. to 266° F.]. The heating-up phase can be followed by aholding phase during which the preform is kept at an elevatedtemperature. For reasons of achieving sufficient mechanical stability,the extraction is generally conducted in such a way that the preformretains only a residue of auxiliaries that does not impair thesubsequent sintering.

[0018] The effect of the treatment gas in terms of removing theauxiliaries is improved if the treatment gas is imparted with a firstflow direction, whereby the treatment gas flows around the preform in afirst flow direction during the heating-up phase, and the flow directionis changed at least once during the course of the heating-up phase. Thiscan be explained by the high density of the supercritical treatment gasthat lies in the range of the specific density of the auxiliaries to beextracted. Due to the temperature rise, the density ratio of thetreatment gas to the auxiliaries to be extracted changes over the courseof the heating-up phase. At the beginning of the heating-up phase, thatis to say, at low temperatures, the density of the treatment gas ishigher than that of the auxiliaries. Preferably, the flow around thepreform is from the bottom to the top so that the treatment gas caneasily displace the auxiliaries upwards and discharge them from theextractor.

[0019] Thus, a mode of operation is preferred in which the flow aroundthe preform is from the bottom to the top in a first time period of theheating-up phase, and from the top to the bottom in a second time periodof the heating-up phase. After a reversal of the density ratios at ahigher temperature, the auxiliaries are removed more efficiently fromthe preform and from the extractor by the treatment gas, which is nowflowing from top to bottom. The temperature above which a reversal ofthe density ratio occurs depends on the internal pressure in theextractor.

[0020] The total duration of the extraction depends essentially on thewall thickness of the preform. It has proven to be advantageous to setthe treatment duration of the preform in the range from one hour tothree hours.

[0021] Propane, nitrous oxide or carbon dioxide have proven to beespecially effective for use as the treatment gas in the sense of theinvention. These treatment gases stand out for their good solventproperties vis-á-vis the conventional binders and plasticizers; they areinexpensive and relatively easy to handle.

[0022] It has proven to be advantageous to extract the auxiliaries whilethe preform is lying on a carrier made of a porous material. Shouldliquid auxiliaries emerge from the preform, they can be adsorbed orabsorbed by the porous carrier material, thus preventing soiling ordamage of the extractor or to other preforms. The carrier can bedesigned, for example, in the form of a plate made of porous ceramics orof porous sintered metal.

[0023] When it comes to the extractor, the above-mentioned technicalobjective is achieved in that it has a pressure-tight housing that isprovided with at least one lateral limiting wall and that encloses atreatment chamber for receiving a carrier element which is loaded with alarge number of preforms, whereby there is at least one inlet for atreatment gas and at least one outlet for a waste gas, and with alockable loading and removal opening for the preforms, said openingbeing configured in the lateral limiting wall as a passage for thecarrier element.

[0024] Since the loading and removal opening is configured as a passagefor the carrier element, the entire carrier element can be insertedthrough this opening into the extractor and taken out again as well. Thepreforms can already be arranged on the carrier element before theloading of the extractor. After the extraction has been completed, thepreforms are taken out of the extractor together with the carrierelement and, lying on the carrier element, they can be conveyed toanother processing device—for instance, a sintering oven.

[0025] Since the removal opening is configured in the lateral limitingwall, the carrier element can be inserted into the extractor by slidingit essentially horizontally, that is to say, without overcoming anymajor differences in height.

[0026] In a preferred embodiment of the extractor according to theinvention, the housing is configured as a hollow cylinder with a closedfront end, whereby the loading and removal opening is provided in thearea of one front end. The two front ends form lateral limiting walls asdefined by/.in the sense of the invention, whereby at least in the areaof the one front end, the loading and removal opening is provided. Acylinder with a circular diameter is especially well-suited as thepressure tank. Here, the cylinder surface contributes to the laterallimitation of the treatment chamber and, at the same time, forms theupper and lower limiting wall.

[0027] Another improvement is attained when the treatment chamber has atleast two inlets and at least two outlets. An extractor that has beenmodified in this manner is especially suitable for use in the variantdescribed above of the method according to the invention in which theflow direction is reversed during the extraction.

[0028] Below, the invention will be explained in greater depth withreference to an embodiment and a drawing. The single figure shows:

[0029] FIG. 1 a device for carrying out the method according to theinvention in a schematic representation.

[0030] The device shown in FIG. 1 serves for the extraction (hereinafteralso referred to as binder removal) of preforms using supercriticalcarbon dioxide. The carbon dioxide is conveyed in a circulation system.The device comprises an extractor 1, a separator 2, and a CO₂ tank 3 forholding carbon dioxide.

[0031] In the CO₂ tank 3, the carbon dioxide is present in a liquidphase 4 and in a gas phase 5. In order to liquefy the carbon dioxide,there is a condenser 6 inside the CO₂ tank 3. From the CO₂ tank 3, thereis a gas line that selectively runs via an outlet 7,.a supercooler 8, aCO₂ pump 9 and a heat exchanger 10 either to an upper inlet 11 or to alower inlet 12 of the extractor 1.

[0032] The extractor 1 has the shape of a hollow cylinder, whereby inthe representation of FIG. 1, the cylinder axis 29 runs horizontally. Inthe area of one of the front ends, there is a filling and removalopening that can be sealed pressure-tight by means of a flap 30. Insidethe extractor 1, there is a mobile rack 15 with several intermediatetrays on which a large number of preforms 13 are arranged for purposesof binder removal. The preforms 13 lie on plates 14 made of poroussintered metal. Through the filling and removal opening, the rack 15 canbe inserted into the extractor and taken out of it.

[0033] Inside the extractor 1, there is also a heating device (not shownin the figure). In an alternative embodiment, the extractor 1 issurrounded by the heating device. The extractor can be connected to theseparator 2 either by means of an upper outlet 16 or a lower outlet 17and by a right-angle valve 18.

[0034] The high-pressure side 19 of the right-angle valve in FIG. 1 isindicated with a smaller triangle and the low-pressure side 20 isindicated with a larger triangle. The right-angle valve 18 is regulatedby a pneumatic device 21 and a pressure regulator 22. The pressureregulator 22 is connected via a pressure line 23 to the high-pressureside 19 of the right-angle valve.

[0035] The separator 2, in which the binder and the plasticizer areseparated from gaseous carbon dioxide, is surrounded by a temperatureregulator 24. The components that condense out of the carbon dioxideform an extract 25 that consists essentially of waxes, paraffins andresins and that can be drained via a connection piece 26. The gas space27 above the extract 25 is connected via a gas line 28 to the gas phase5 of the CO₂ tank 3.

[0036] Below, the method according to the invention for the gentleextraction of auxiliaries from preforms using supercritical carbondioxide will be explained in greater depth with reference to FIG. 1:

[0037] The extractor 1 is loaded with a batch of preforms 14 in that thepreforms 14 to be treated are arranged on the rack 15 outside of theextractor 1 and the rack is subsequently slid horizontally into theextractor 1.

[0038] The preforms are cast parts that have been made of a mixture ofcarbonyl iron powder and auxiliaries, such as binders and plasticizers,by means of powder injection molding. The auxiliaries are combined belowunder the term “binders”. Relative to the total weight, the proportionof binders is 6% to 9% by weight. The various binder fractions arecontained in differing quantitative proportions. The binder fractionsdiffer in terms of their melting temperature and their solubility insupercritical carbon dioxide.

[0039] In the CO₂ tank 3, carbon dioxide is kept at a temperature ofabout 20° C. [68° F ] and at a pressure in the range from 55 bar to 60bar. In this process, a phase equilibrium is established with theformation of the gas phase 5 and the liquid phase 4. Liquid carbondioxide is continuously removed from the liquid phase 4 and conveyed viathe supercooler 8 and the CO₂ pump 9 under a pressure of 200 bar to 500bar as carbon dioxide to the heat exchanger 10. As a rule, a pressure of350 bar is established in the extractor 1.

[0040] The binder removal according to the invention comprises aheating-up phase during which the temperature of the stream ofsupercritical carbon dioxide that is continuously fed through theextractor 1 is slowly raised. The temperature of the carbon dioxide isregulated in the heat exchanger 10. In the embodiment, an incrementaltemperature increase is established according to the following table:TABLE 1 Temperature course during the extraction with supercritical CO₂No. Duration [min] Temperature [° C./° F.] 1 25 30/86  2 15 50/122 3 1570/158 4 15 90/194 5 20 120/248 

[0041] As an alternative to the incremental temperature increase, thetemperature can also be increased continuously. In an accordinglymodified embodiment, the temperature of the supercritical carbon dioxideis regulated by the heat exchanger 10, whereby a continuous temperatureincrease from 30° C. to 120° C. [86° F. to 248° F.] at a heating rate of1° C. [1.8° F.] per minute is established.

[0042] From the heat exchanger 10, the continuous stream ofsupercritical carbon dioxide enters the extractor 1, flows around theporous preforms 14, thus dissolving components of the binder containedtherein. The amount of the carbon dioxide to be used depends on theweight of material from which the binder is to be removed. In theembodiment, 10 kg of supercritical carbon dioxide per kilogram of thestarting weight of the preforms is used.

[0043] The gradual temperature increase brings about a steadydissolution of the various binder fractions so that sufficientdimensional stability of the preforms 14 is ensured during the binderremoval. Moreover, a liquefaction of the binder is avoided in thatlow-melting fractions are already dissolved before their meltingtemperature has been reached. Any liquid that nevertheless emerges isabsorbed by the porous plates 13.

[0044] Due to the temperature increase, over the course of theheating-up phase, there is a change in the destiny ratio of thesupercritical carbon dioxide to the binder to be extracted. Therefore,at the beginning of the heating-up phase, the stream of supercriticalcarbon dioxide flows from the lower inlet 12 to the, upper outlet 16. Asa result of the flow direction that is established in this manner,binder components whose density is less than that of the supercriticalcarbon dioxide are displaced upwards and discharged from the extractor.After a reversal of the above-mentioned density ratio, which occurs as afunction of the selected binder system at an internal pressure of 350bar at a temperature in the range from 70° C. to 90° C. [158° F. to 194°F.], the flow direction is changed in that the carbon dioxide stream nowflows from the upper inlet 11 to the lower outlet 17 so that the bindercomponents that now have a higher density can more easily be removed viathe lower outlet 17.

[0045] Subsequent to the heating-up phase, the preforms 14 are kept at atemperature of 120° C. [248° F.] for a holding time of 20 minutes,whereby the supercritical carbon dioxide continues to flow around them.The extraction is terminated when only a residue of binders is presentin the preforms 14, which is necessary for the stability of the preforms14 until they are sintered. This residual fraction amounts to about 20%by weight. Consequently, in the embodiment, the extraction is completedafter 90 minutes.

[0046] The binder components discharged from the extractor 1 reach theseparator 2 and condense or crystallize there as a result of cooling offthrough the expansion of the carbon dioxide. The purified carbon dioxideis then conveyed back to the CO₂ tank 3.

[0047] Through the use of supercritical carbon dioxide as the extractionagent and due to its good dissolving and transport properties, which canbe precisely regulated by means of the temperature control in the heatexchanger 10, the binder removal times are shortened to just a fewhours. The method according to the invention allows the extract 25 to beused again, thus alleviating the negative impact on the environment. Thesolvent carbon dioxide is also conveyed in a circulation system.

[0048] After the binder removal, the flap 30 is opened, the rack 15 isslid out of the extractor 1 and the preforms 14 resting on it are takento a sintering oven (not shown in FIG. 1) for the finishing of thedesired components. The sintering oven is likewise configured to receivethe rack 15 so that the treated preforms 14 do not have to betransferred. They are subsequently sintered at a temperature of 1000° C.to 1350° C. [1832° F. to 2462° F.].

1. A method for producing a component from a pulverulent startingmaterial by mixing the powder together with auxiliaries, comprisingbinders, resulting in the formation of a free-flowing mass, after whicha preform is produced from the mass by means of powder injectionmolding, then some of the auxiliaries are extracted in that the preformis heated in a tank and exposed to a solvent stream, after which thepreform is sintered, thus giving rise to the component, characterized inthat the extraction comprises a heating-up phase during which thetemperature of the preform (14) is continuously or incrementallyincreased and a solvent in the form of a compressed, supercriticaltreatment gas flows around the preform (14).
 2. The method according toclaim 1, characterized in that, during the heating-up phase, a heatingrate is set/./established in the range from 0.1° C./min to 5° C./min[0.2° F./min to 9° F./min], preferably in the range from 0.5° C./min to2° C./min [0.9° F./min to 3.6° F./min].
 3. The method according to claim1 or 2, characterized in that, during the heating-up phase, thetemperature of the preform is raised to the range from 20° C. to 150° C.[68° F. to 302° F.], preferably from 30° C. to 130° C. [86° F. to 266°F.].
 4. The method according to one of the preceding claims,characterized in that the treatment gas is imparted with a first flowdirection, and the treatment gas flows around the preform (14) in theflow direction during the heating-up phase, and in that the flowdirection is changed at least once during the course of the heating-upphase.
 5. The method according to claim 4, characterized in that theflow around the preform is from the bottom to the top in a first timeperiod of the heating-up phase, and from the top to the bottom in asecond time period of the heating-up phase.
 6. The method according toone of the preceding claims, characterized in that the treatmentduration of the preform (14) is set in the range from one hour to threehours.
 7. The method according to one of the preceding claims,characterized in that propane, nitrous oxide or carbon dioxide are usedas the treatment gas.
 8. The method according to one of the precedingclaims, characterized in that the preform (14) lies on a carrier made ofa porous material during the extraction of the auxiliaries.
 9. Anextractor for carrying out the method according to one or more of claims1 to 8, with a pressure-tight housing (1) that is provided with at leastone lateral limiting wall and that encloses a treatment chamber forreceiving a carrier element (15) which is loaded with a large number ofpreforms (14), whereby there is at least one inlet (11; 12) for atreatment gas and at least one outlet (16; 17) for a waste gas, and witha lockable loading and removal opening (30) for the preforms (14), saidopening being configured in the lateral limiting wall as a passage forthe carrier element (15).
 10. The extractor according to claim 9,characterized in that the housing (1) is configured as a hollow cylinderwith a closed front end, whereby the loading and removal opening (30) isprovided in the area of one front end.
 11. The extractor according toclaim 9 or 10, characterized in that the treatment chamber has at leasttwo inlets (11; 12) and at least two outlets (16; 17).